Together with the ethnic Russians, the Soviet government settled many people from other parts of the Soviet Union in cities like Vilnius and Klaipėda. These are Belarussians, Ukrainians, Armenians, Azeri, Moldovans, people from the Central Asia.

Neither of these ethnicities (with the exception of Belarussians) existed in Lithuania prior to 1940 in numbers larger than 50 individuals. People from all them share certain similar traits. Despite different origins, they are quite frequently using the Russian language at home and they likely speak Russian better than Lithuanian. Due to this reason, they are often grouped together with Russians under the term "Russophone".

Politically the "Russophones" typically identify the Soviet Union positively and are more likely to be atheists than most other ethnicities.

The above traits are more pronounced in some ethnicities (Ukrainians, Belarussians, interethnic Russophone families) and are less pronounced in some others (Georgians for instance). 55% of the Belarusian community speak Russian natively (only 35% speaks Belarusian). The same is true for 66% Jews, 56% Ukrainians, 47% Tatars, 38% Estonians, 36% Armenians, 36% Georgians, 31% Moldovans and 27% Azeris.

The Russophone community was partly created by the Soviet policies which did not establish any non-Russian Soviet language schools, theaters or media outside of that language's titular homeland, forcing the increasing internal migrants (and some historical minorities) to use the wide network of Russian institutions instead.

Due to a wide knowledge of the Russian language in Lithuania people of the former Soviet Union (especially Belarus, Ukraine, Georgia) continue to immigrate, albeit in lower numbers than in 1940s-1980s. This was especially prevalent before the economic downtime of 2008 when they used to take construction, truck driving, and other similar jobs. New immigrants are often less Russified, some of them are actually anti-Russian. Despite immigration, the "Russophone" share decreases with assimilation into Lithuanian and Russian communities: only ~5% of Lithuania's Belarusians and Ukrainians are younger than 19.

Around 2% of Lithuania’s population are from one of these ethnicities (excluding the ethnic Russians).

You are correct that Belarusians have a longer history in Lithuania than the other ethnicities mentioned here and perhaps they should be separated.

The current form of the article was chosen because the modern-day Belarusian community in Lithuania is mainly descending of these more recent arrivals rather than of the historic Belarusian community.

In 1923, Belarusians made 0,2% of Lithuanian population (excluding Vilnius region), and in 1931 Belarusians made 0,9% of the population in Vilnius city. It could be roughly calculated that in total, there were approximately 0,3% of Belarusians in the territories of modern-day Lithuania. In comparison, by 1989 this number was 1,7% (six times larger).

Moreover, since 1940, the policies toward Belarusians of the government (Soviet) were similar to the policies towards other ethnicities lumped here, i.e. russification was rampant and expected, no cultural institutions were permitted to exist. Therefore, the difference between modern-day ex-Soviet communities not that big, with most of them going to Russian (some now Lithuanian) schools and significant share speaking Russian at home, etc.

There was no name Belarus in 14 century.The Moscowitz called them Litva people(Fedorovska letopis book)The majority of population of Great Duchy of Lithuania in 14 -16 century was (Old Ruthenians) or Belorussians & Ukrainians- over 2 mln. Population of ethnic Zhemaytians in 14-15 century was around 400 thousands only.The state language mainly was Old Ruthenian (Belorussian).
The historical Statut (Law book ) was written in Old Ruthenian language.
This rare book was purchased by Belarus museum at auction in Great Britain in recent years. Regards

Grand Duchy of Lithuania did indeed have a Slavic population majority at its further territorial extent, from the early 1400s to 1569 Union of Lublin. It also used an Eastern Slavic language as the written language. It is not the same as the modern-day state language, however: at the time, only a few people (e.g. scribes) knew writing and therefore written languages were not the ones spoken by the leaders usually or the ones spoken in the capital (e.g. entire Western Europe used Latin language as a written language in the Medieval times).

The areas of modern-day Lithuania (also Kaliningrad Oblast, northeast Poland, and northwest Belarus) were inhabited by ethnic Lithuanians (i.e. Balts, not Slavs). Vilnius had some East Slavic (Old Ruthenian) population in the 14th-16th centuries (there are still 4 remaining Orthodox churches dating to that era on the eastern side of the Old Town which used to be an Eastern Slavic district of Vilnius at the time). However, even in Vilnius, the majority of the population was Lithuanian and for every Orthodox church, there were numerous Catholic churches. Gradually, both Lithuanian and Eastern Slavic inhabitants of Vilnius adopted Polish language and customs, and by the 19th century, Vilnius was largely Polish-speaking. A minority of them have since reverted to the Lithuanian language, especially during the National Revival ~1900.

Samogitians were (and still are) a unique group living in Western Lithuania. They had a separate region “Duchy (Elderate) of Samogitia” at the time. Currently, they are usually considered to be a part of Lithuanian nation, although some of them may disagree.